1. Field of the Invention
The present invention relates generally to an automotive suspension system which has variable suspension characteristics depending upon the vehicle driving condition in order to achieve both vehicle driving stability and riding comfort. More specifically, the invention, relates to an automotive suspension system which discriminate vibration caused due to vehicle body attitude change from vibration caused due to vibration, such as road shock, input through a vehicular wheel in order to appropriately adjust damping characteristics of the suspension system.
2. Description of the Background Art
In the recent automotive technologies, respective automotive components have been required to exhibit high performance and high response. In case of a suspension system, a high level of riding comfort and driving stability has been required. In order to achieve both high level riding comfort and high level driving stability, a high response against vibration input to the suspension system has been required.
One of the typical variable damping force suspension systems has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-85210. In the disclosed system, a piezoelectric element is disposed in each shock absorber in each suspension system for detecting variation of fluid pressure in the shock absorber. A control unit is provided for receiving an input indicative of the fluid pressure provided from the piezoelectric element. The control unit outputs a controlled voltage to the piezoelectric element for switching an operation mode of the shock absorber at least between a SOFT mode in which a smaller damping force is to be generated in response to vibration input and a HARD mode in which a greater damping force is to be generated in response to vibration input.
In general, the control unit is responsive to low frequency input vibration which induces an attitude change of the vehicle body to switch the operational mode of the shock absorber into a HARD mode for a given period of time. While the shock absorber is maintained at the HARD mode, the piezoelectric element maintains operation as an actuator for maintaining the HARD mode operation of the shock absorber. Such manner of control of suspension characteristics may be effective in terms of suppression of vehicular attitude change, such as rolling and/or pitching. However, on the other hand, even in response to road shock input from a vehicular wheel, it is likewise desirable to adjust the suspension characteristics depending upon the nature of road shock in order to satisfactorily achieve both riding comfort and driving stability. Therefore, the prior proposed variable damping characteristic suspension systems for automotive vehicle are not at all satisfactory.
Furthermore, in the prior proposed system, while it is active as the actuator, the piezoelectric element can not monitor fluid pressure.
In the modern technology of suspension control, it has been considered desirable that varying the damping characteristics of a shock absorber between a piston compression stroke in response to a bounding motion between the vehicle body and a road wheel and a piston expansion stroke in response to a rebounding motion between the vehicle body and the road wheel in order to obtain better vibration stabilizing performance. Therefore, it is desirable to adjust the damping characteristics of the shock absorber depending upon the mode of piston action. In order to realize this, it is essential to detect the piston action mode on the basis of a variation of the fluid pressure in the shock absorber. However, as set forth above, the piezoelectric element is held inoperative as the fluid pressure sensing element while the shock absorber is maintained at the HARD mode.
This may cause a problem in damping shocks. For example, when the damping characteristic in the HARD mode is set to generate a relatively great damping force in response to vibration input, a damping force generated in response to piston compression mode action can amplify the input vibration. This tendency may be significant for the second and subsequent vibration cycles. This clearly degrades the vibration stabilizing performance of the vehicle to provide a rough ride feeling.
In addition, the prior proposed suspension system operates substantially in a passive manner to detect the input vibration based on variation of fluid pressure in the shock absorber. Namely, until the vehicle driving condition is changed to require switching of suspension characteristics, the suspension will never operate to switch suspension characteristics. Therefore, when a criterion of the fluid pressure to switch the suspension mode from a SOFT mode to a HARD mode is set at a relatively high value in order to provide better riding comfort, the switch from the SOFT mode to the HARD mode tends to delay to cause bottoming due to low response characteristics particularly at the initial stage of switching of the suspension mode.